1. Field of the Invention
The invention concerns a cathode of the type having a cathode head in which at least one emitter is arranged that emits electrons upon application of a heating voltage.
2. Description of the Prior Art
In known cathodes of the above type, the is lies at the same potential as the cathode head and can be switched to a more negative potential by the application of a blocking or reverse voltage, so the electrons that are thermally released from the emitter given a heating voltage applied to the emitter are prevented from exiting the cathode head. Known cathodes have filament (helical) emitters (filaments) or surface emitters and are used in x-ray tubes, for example. If the blocking voltage is not applied, the emitted electrons are accelerated in the direction of the anode. When the electrons strike the anode, x-ray radiation is generated in the surface of the anode.
A cathode with a filament emitter is known from DE 199 55 845 A1, for example. Cathodes that have surface emitters are described in DE 199 14 739 C1 and DE 10 2008 011 841 A1, for example.
In radiography, or tomography with x-ray radiation, the contrast of the x-ray exposures is better the lower the energy of the x-ray radiation. The exposure of the x-ray acquisition can be regulated by the exposure duration or by the intensity of the x-ray radiation. Since image artifacts occur in most medical examinations with a long exposure duration due to movement of the patient, the desired exposure is regulated by the intensity of the x-ray radiation that is generated by the impact of an electron beam (generated by an emitter) on the anode.
An increase of the intensity of the electron beam leads to an increased repulsion of the electrons generated by the emitter among one another (volume charge). This increased volume charge means that the focusing of the electrons that is produced by the cathode head is partially canceled. The electron beam is thereby expanded and the geometry of the focal spot on the anode is degraded.
Since the size of the electron beam striking the anode (focal spot size or focal spot geometry) in most cases strongly depends in most cases on the intensity of the electrons emitted by the emitter, and the focal spot geometry strongly influences the resolution capability of the x-ray beam, the resolution capability of the x-ray beam and the total quality of the x-ray exposure are strongly affected.
In order to influence the focal spot geometry and the focal spot position, it is known from DE 197 45 998 A1 to focus the electron beam by magnetic or electrical lens systems.
Furthermore, by means of an external voltage source it is known to generate a volume charge compensation through a potential difference between cathode head and emitter.